Nickel metal hydride (NiMH) batteries (cells) employ a negative electrode (anode) capable of reversible electrochemical storage of hydrogen. NiMH cells usually employ a positive electrode (cathode) comprising nickel hydroxide active material. The negative and positive electrodes are spaced apart in an aqueous alkaline electrolyte. Upon application of an electrical potential across the cell, hydrogen ions from the electrolyte combine with electrons and diffuse into the bulk of the hydrogen storage alloy to form a metal hydride. Upon discharge, stored hydrogen is released from the metal hydride as protons and electrons. Water is reformed from the protons and hydroxyl ions in the electrolyte.
NiMH batteries are employed in a wide variety of end uses, for instance in portable consumer products such as digital cameras, cell phones, etc., electric and hybrid vehicle applications and industrial standby applications.
The charge and discharge reactions that take place at the nickel hydroxide positive electrode are:Ni(OH)2+OH−⇄NiOOH+H2O+e−
Nickel hydroxide oxidizes to nickel oxyhydroxide upon charging; upon discharge nickel oxyhydroxide is reduced back to nickel hydroxide.
Nickel cadmium (NiCd) cells also employ a positive electrode comprising nickel hydroxide active material. Other types of nickel based cells include nickel hydrogen, nickel zinc and nickel iron.
The electrochemical reaction at the cathode is known to involve the transfer of one electron, between the stable Ni2+ β phase of Ni(OH)2 and the Ni3+ β phase of NiOOH. The theoretical specific capacity of nickel hydroxide active material based on this is 289 mAh/g.
Transfer of more than one electron per Ni atom would lead to higher specific capacity. Nickel hydroxide material capable of transferring more than one electron is mentioned for instance in U.S. Pat. Nos. 5,348,822, 5,569,563, 5,567,549 and 6,228,535. In these prior arts, more than one electron transfer per Ni atom may involve a highly oxidized γ NiOOH phase.